Dehydration method and dehydration apparatus

ABSTRACT

A dehydration method is a dehydration method for selectively separating water from a mixture that contains water, and the method includes a step of supplying the mixture to a supply side space of a zeolite membrane having an ERI structure, and a step of making a pressure difference between the supply side space and a permeation side space of the zeolite membrane having an ERI structure.

TECHNICAL FIELD

The present invention relates to a dehydration method and a dehydrationapparatus.

BACKGROUND ART

Organic membranes and inorganic membranes are conventionally used forseparating water from (dehydrating) a mixture that contains water.

However, organic membranes are inferior in terms of heat resistance andchemical resistance, and therefore dehydration methods are proposed inwhich inorganic membranes, such as an A-type zeolite membrane (seeDevelopment of Membrane Aided Reactor, Mitsui Zosen Technical Review,February 2003, No. 178, 115-120., for example) and a T-type zeolitemembrane (see Y. Cui et al., Zeolite T membrane: preparation,characterization, pervaporation of water/organic liquid mixtures andacid stability, Journal of Membrane Science, 2004, 236, 17-27, forexample), are used as separation membranes.

SUMMARY

However, there is a risk that, in the dehydration method described in“Development of Membrane Aided Reactor”, the A-type zeolite membranewill partially dissolve in water over a long period of use, and itsdehydration performance will degrade. The T-type zeolite membranedescribed in “Zeolite T membrane: preparation, characterization,pervaporation of water/organic liquid mixtures and acid stability” hashigher acid resistance when compared with A-type zeolite membranes, butthis membrane also contains a zeolite having an OFF structure thatincludes pores larger than those of an ERI zeolite. Therefore, there isa risk that components that should not permeate will permeate throughpores of the OFF structure, and furthermore, the membrane is unlikely tobe dense and accordingly there is a risk that separation performancewill not be sufficiently exhibited. Therefore, there are demands for adehydration method that can suppress degradation of dehydrationperformance even for long periods of use.

The present invention was made in view of the above circumstances, andan object of the present invention is to provide a dehydration methodand a dehydration apparatus that can suppress degradation of dehydrationperformance.

A dehydration method according to the present invention is a dehydrationmethod for selectively separating water from a mixture that containswater, and the method includes a step of supplying the mixture to asupply side space of a zeolite membrane having an ERI structure, and astep of making a pressure difference between the supply side space and apermeation side space of the zeolite membrane having an ERI structure.

According to the present invention, a dehydration method and adehydration apparatus that can suppress degradation of dehydrationperformance can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the configuration of a dehydrationapparatus.

FIG. 2 is a cross-sectional view of a membrane structure.

FIG. 3 is a SEM image showing a cross section of a porous support and anERI membrane.

FIG. 4 is a SEM image showing a surface of an ERI membrane.

FIG. 5 is a schematic diagram showing the configuration of an ERIcrystal.

FIG. 6 is a schematic diagram showing a method for manufacturing an ERImembrane.

DESCRIPTION OF EMBODIMENTS

Dehydration Apparatus

The following describes, with reference to the drawings, one example ofa dehydration apparatus that is used for carrying out a dehydrationmethod for selectively separating water from a mixture containing water.In the present specification, “dehydration” means selectively separatingwater. “Selectively separating water” includes not only separating andtaking out water of 100% purity from a mixture, but also separating andtaking out a solution or gas that has a higher water content than thatof the mixture.

FIG. 1 is a schematic diagram showing the entire configuration of adehydration apparatus 100 according to the present embodiment.

The dehydration apparatus 100 includes an accommodation portion 10, acirculation pump 20, a heater 30, a separation vessel 40, a trappingportion 50, a pressure reducing apparatus 60, a circulation path 70, anda permeation path 80. The accommodation portion 10, the circulation pump20, the heater 30, and the separation vessel 40 are arranged on thecirculation path 70. The trapping portion 50 and the pressure reducingapparatus 60 are arranged on the permeation path 80.

The accommodation portion 10 accommodates a mixture 11 to be processed.The mixture 11 is circulated through the circulation path 70 to theaccommodation portion 10. The mixture 11 contains water and componentsother than water.

The mixture 11 may contain water and organic compounds. Examples oforganic compounds include alcohols, phenols, aldehydes, ketones,carboxylic acids, sulfonic acids, ethers, esters, amines, nitriles,straight-chain saturated hydrocarbons, branched saturated hydrocarbons,cyclic saturated hydrocarbons, chain unsaturated hydrocarbons, aromatichydrocarbons, nitrogen-containing compounds, sulfur-containingcompounds, and halogen derivatives of hydrocarbons. Examples of alcoholsinclude methanol, ethanol, isopropanol, ethylene glycol, and butanol.Examples of ketones include acetone and ethyl methyl ketone. Examples ofcarboxylic acids include formic acid, acetic acid, butyric acid,propionic acid, oxalic acid, acrylic acid, and benzoic acid. Examples ofaromatic hydrocarbons include toluene and benzene. The mixture 11 maycontain only one component other than water or contain two or morecomponents other than water.

The circulation pump 20 circulates the mixture 11 through thecirculation path 70 by discharging the mixture 11 to the separationvessel 40 side. It is preferable that the supply fluid velocity of themixture 11 supplied to the separation vessel 40 is 1.5 m/s or more and3.0 m/s or less in cells 43, which will be described later.Alternatively, it is preferable that the Reynolds number caused by thesupply fluid velocity of the mixture 11 supplied to the separationvessel 40 is 2000 or more and 10000 or less.

The heater 30 heats the mixture 11 circulated through the circulationpath 70 to a temperature that is suitable for dehydration performed inthe separation vessel 40. The temperature of the mixture 11 supplied tothe separation vessel 40 is preferably from 50° C. to 130° C., and morepreferably from 55° C. to 110° C., in order to efficiently perform adehydration process.

The separation vessel 40 includes a housing portion 41 and a membranestructure 42. The housing portion 41 accommodates the membrane structure42. The material of the housing portion 41 is not particularly limited,and can be determined as appropriate in accordance with characteristicsof the mixture 11, for example. If the mixture 11 contains acid, forexample, the housing portion 41 can be made of glass, stainless steel,or the like.

The interior space of the housing portion 41 is sectioned into a supplyside space 4S and a permeation side space 4T by a zeolite membrane 45having an ERI structure and included in the membrane structure 42, whichwill be described later (see FIG. 2). That is, the zeolite membrane 45having an ERI structure and included in the membrane structure 42separates the supply side space 4S and the permeation side space 4T fromeach other. The mixture 11 is supplied to the supply side space 4S. Outof the components of the mixture 11, a membrane-permeating substance 12permeated through the zeolite membrane 45 having an ERI structure andincluded in the membrane structure 42 flows into the permeation sidespace 4T. The membrane-permeating substance 12 is water or a solution orgas in which water is concentrated. The configuration of the membranestructure 42 will be described later.

Note that pressure sensors (not shown) are connected to the separationvessel 40, and the pressure in the supply side space 4S and the pressurein the permeation side space 4T can be detected by the pressure sensors.

The trapping portion 50 is connected to the separation vessel 40 via thepermeation path 80. When a dehydration process is carried out, theinside pressure of the trapping portion 50 can be reduced, and further,the pressure in the permeation side space 4T of the housing portion 41can be reduced to a predetermined pressure, as a result of the pressurereducing apparatus 60 operating.

The trapping portion 50 is made of a material that can withstandpressure applied during a pressure reducing operation. The trappingportion 50 can be made of glass, stainless steel, or the like, forexample.

A refrigerant may be used in the trapping portion 50 in order to cooland trap vapor of the membrane-permeating substance 12 flowing into thetrapping portion 50. The refrigerant can be selected as appropriatedepending on the type of the membrane-permeating substance 12 and theinside pressure of the trapping portion 50. Examples of refrigerantsthat can be used include liquid nitrogen, ice water, water, antifreezeliquid, dry ice (solid carbon dioxide), a combination of dry ice andethanol (or acetone or methanol), and liquid argon.

However, the trapping portion 50 is not limited to the structure shownin FIG. 1, and is only required to be capable of trapping themembrane-permeating substance 12 while the pressure in the permeationside space 4T of the housing portion 41 is reduced to a predeterminedpressure.

The pressure reducing apparatus 60 is one example of a “pressurechanging apparatus” for making a pressure difference between the supplyside space 4S and the permeation side space 4T. In the presentembodiment, the pressure reducing apparatus 60 reduces the pressure inthe permeation side space 4T to a predetermined pressure or a lowerpressure. “Reducing the pressure” includes reducing partial pressure ofthe membrane-permeating substance 12 in the permeation side space 4T. Awell-known vacuum pump can be used as the pressure reducing apparatus60, for example.

Note that a pressure controller for adjusting the pressure in thepermeation side space 4T may also be provided on the permeation path 80.

Membrane Structure

FIG. 2 is a cross-sectional view of the membrane structure 42.

The membrane structure 42 includes a porous support 44 and the zeolitemembrane 45 having an ERI structure. The zeolite membrane 45 having anERI structure is constituted by zeolite crystals 46 having an ERIstructure.

In the following description, the zeolite membrane 45 having an ERIstructure is abbreviated as an “ERI membrane 45”, and a zeolite crystal46 having an ERI structure is abbreviated as an “ERI crystal 46”.

1. Porous Support 44

The porous support 44 supports the ERI membrane 45. The porous support44 has chemical stability to an extent that the ERI membrane 45 can beformed (crystallized, applied, or deposited) on a surface of the poroussupport 44 in the form of a membrane.

The porous support 44 is a ceramic sintered body. Alumina, silica,mullite, zirconia, titania, yttria, silicon nitride, silicon carbide,ceramic sand, cordierite, and the like can be used as the aggregate ofthe porous support 44. The porous support 44 may contain a bindingmaterial. A glass material containing silicon (Si), aluminum (Al),titanium (Ti), and the like can be used as the binding material. Thecontent of the binding material may be set to be 20 vol % or more and 40vol % or less, but is not limited thereto.

In the present embodiment, the porous support 44 has a monolith-shape. Amonolith-shape refers to a shape having a plurality of cells 43 providedin the longitudinal direction, and includes a honeycomb shape. However,the porous support 44 is only required to have a shape with which themixture 11 to be processed can be supplied to the ERI membrane 45. Forexample, the porous support 44 may have a flat plate-like shape, atubular shape, a cylindrical shape, a columnar shape, or a prismaticcolumn-like shape. Surface roughness (Ra) of the porous support 44 ispreferably 0.1 μm to 2.0 μm, and more preferably 0.2 μm to 1.0 μm. Ra ofthe porous support 44 can be measured using a stylus surface roughnessmeasurement device.

If the porous support 44 has a monolith-shape, the length thereof in thelongitudinal direction can be set to 100 to 2000 mm, and the diameterthereof in the radial direction can be set to 5 to 300 mm, but there isno limitation thereon. If the porous support 44 has a monolith-shape, itis possible to form 30 to 2500 cells 43 having a diameter of 1 to 5 mmin the porous support 44. The distance between central axes of adjacentcells 43 can be set to 0.3 mm to 10 mm, for example. If the poroussupport 44 has a tubular shape, the thickness of the porous support 44can be set to 0.1 mm to 10 mm, for example.

The porous support 44 is a porous body having multiple open pores. Anaverage pore size of the porous support 44 need only be a size at whichthe membrane-permeating substance 12 (mainly water) in the fluid mixturethat has permeated through the ERI membrane 45 can pass through pores.The permeation amount of the membrane-permeating substance 12 can beincreased by increasing the average pore size of the porous support 44.The strength of the porous support 44 can be increased by reducing theaverage pore size of the porous support 44. The average pore size of theporous support 44 can be 0.01 μm or more and 5 μm or less, for example.The average pore size of the porous support 44 can be measured,depending on the size of pores, using a mercury intrusion method, anair-flow method described in ASTM F316, or perm porometry. The porosityof the porous support 44 is not particularly limited, and can be 25% to50%, for example. With regard to a cumulative volume distribution of thepore size of the porous support 44, D5 can be 0.1 μm to 50 μm, forexample, D50 can be 0.5 μm to 70 μm, for example, and D95 can be 10 μmto 2000 μm, for example.

An average particle size of the porous support 44 is not particularlylimited, and can be 0.1 μm or more and 100 μm or less, for example. Theaverage particle size of the porous support 44 refers to an arithmeticaverage value of the maximum diameters of 30 particles that are measuredthrough cross-sectional observation using a SEM (Scanning ElectronMicroscope). 30 particles to be measured need only be selected in a SEMimage at random.

The porous support 44 may have a monolayer structure in which pores havea uniform size, or a multilayer structure in which pores have differentsizes. If the porous support 44 has a multilayer structure, it ispreferable that the closer a layer is to the ERI membrane 45, thesmaller the average pore size is. If the porous support 44 has amultilayer structure, the average pore size of the porous support 44refers to an average pore size of an outermost layer that is in contactwith the ERI membrane 45. If the porous support 44 has a multilayerstructure, each layer can be constituted by at least one selected fromthe above-described materials, and materials constituting layers may bedifferent from each other.

2. ERI Membrane 45

FIG. 3 is a SEM (scanning electron microscope) image showing a crosssection of the ERI membrane 45. FIG. 4 is a SEM image showing a surfaceof the ERI membrane 45.

The ERI membrane 45 is formed on an inner surface of the porous support44. The ERI membrane 45 is formed into a tubular shape. The space insidethe ERI membrane 45 is the supply side space 4S and the space outsidethe ERI membrane 45 (i.e., the porous support 44 side space) is thepermeation side space 4T. In the present embodiment, the supply sidespace 4S is a cell 43. The permeation side space 4T includes not onlythe exterior space of the porous support 44 but also the inside of theporous support 44.

Thus, one surface of the ERI membrane 45 faces the supply side space 4Sand the other surface of the ERI membrane 45 faces the permeation sidespace 4T. When the mixture 11 is supplied to the supply side space 4S,the mixture 11 comes into contact with the one surface of the ERImembrane 45. When the pressure in the permeation side space 4T isreduced in this state, the membrane-permeating substance 12 contained inthe mixture 11 permeates through the ERI membrane 45. Themembrane-permeating substance 12 is water or a solution or gas in whichwater is concentrated. As described above, the membrane-permeatingsubstance 12 permeated through the ERI membrane 45 is sucked by thepressure reducing apparatus 60 and is trapped in the trapping portion50.

The thickness of the ERI membrane 45 is not particularly limited, andcan be set to 0.1 μm or more and 10 μm or less. The ERI membrane 45preferably has a thickness of 0.3 μm or more, and more preferably has athickness of 0.5 μm or more, in consideration of sufficiently bondingcrystals. The ERI membrane 45 preferably has a thickness of 5 μm orless, and more preferably has a thickness of 3 μm or less, inconsideration of suppressing cracking caused by thermal expansion.Surface roughness (Ra) of the ERI membrane 45 is preferably 5 μm orless, and more preferably 2 μm or less. Ra of the ERI membrane 45 ismeasured by using a confocal laser microscope that can be used forthree-dimensional measurement, obtaining values of Ra in 10 randomlyselected fields of vision of 100 μm square by correcting waviness of theporous support 44, and taking the smallest value of the thus obtainedvalues as the value of Ra.

The ERI membrane 45 is formed in the form of a membrane as a result of aplurality of ERI crystals 46 being linked to each other. Each ERIcrystal 46 is a crystal constituted by a zeolite having an ERIstructure. The ERI structure refers to a type of structure that meetsthe definition of an ERI type structure under the IUPAC structure codesas defined by the Structure Commission of the

International Zeolite Association.

Examples of zeolites constituting ERI crystals 46 include a zeolite inwhich atoms (T atoms) located at centers of oxygen tetrahedrons (TO₄)constituting the zeolite are constituted by Si and Al, an AlPO zeolitein which T atoms are constituted by Al and P (phosphorus), an MAPOzeolite in which T atoms are constituted by magnesium (Mg), Al, and P,an SAPO zeolite in which T atoms are constituted by Si, Al, and P, and aZnAPO zeolite in which T atoms are constituted by zinc (Zn), Al, and P.A portion of T atoms may be substituted by another element.

Each ERI crystal 46 internally has a plurality of oxygen 8-membered ringpores. An oxygen 8-membered ring pore refers to a pore constituted by anoxygen 8-membered ring. An oxygen 8-membered ring is also simplyreferred to as an “8-membered ring”, and is a portion in which thenumber of oxygen atoms constituting the pore framework is eight, andoxygen atoms are linked to the above-described T atoms to form a ringstructure.

Each ERI crystal 46 may contain a metal or metal ion for the purpose ofproviding absorptivity with respect to a specific component. Examples ofsuch a metal or metal ion include one or more selected from the groupconsisting of alkali metals, alkaline earth metals, and transitionmetals. Although specific examples of transition metals include platinum(Pt), palladium (Pd), rhodium (Rh), silver (Ag), iron (Fe), copper (Cu),cobalt (Co), manganese (Mn), and indium (In), there is no limitationthereon.

Here, FIG. 5 is a schematic diagram showing the configuration of an ERIcrystal 46. As shown in FIG. 5, the ERI crystal 46 has a hexagonalcolumn-like shape. It is preferable that the shape of a cross section ofthe ERI crystal 46 that is parallel to a c-plane is a regular hexagon,but there is no particular limitation thereon. If the cross section ofthe ERI crystal 46 has a hexagonal shape, it is possible to obtain amembrane that has high crystallinity and excellent durability, whencompared with a case in which the cross section has an irregular shape,a circular shape, or an elliptical shape, for example.

A hexagonal c-plane ((001) plane) exists at an end face of the ERIcrystal 46. Rectangular a-planes ({h00} planes) exist at side faces ofthe ERI crystal 46.

As shown in FIGS. 3 and 4, ERI crystals 46 stand on a surface of theporous support 44 and the orientation of the ERI crystals 46 is ac-plane orientation. Accordingly, mainly c-planes are exposed at theouter surface of the ERI membrane, and the ERI crystals 46 are joined toeach other mainly at a-planes. With this configuration, bonding betweenthe ERI crystals 46 can be increased, and therefore density of themembrane is increased, and separation performance can be sufficientlyexhibited.

In an X-ray diffraction pattern obtained by irradiation of X-rays to thesurface of the ERI membrane 45 using an X-ray diffraction (XRD) method,the peak intensity of a (002) plane (c-plane) is 0.5 times or more thepeak intensity of a (100) plane (a-plane). This indicates that thec-plane orientation of the ERI crystals 46 is high. Accordingly,separation performance of the ERI membrane 45 can be improved to anextent that allows for its practical use by setting the peak intensityof the (002) plane to 0.5 times or more the peak intensity of the (100)plane.

In the X-ray diffraction pattern, the peak intensity of the (002) planeis preferably 0.9 times or more the peak intensity of the (100) plane,and more preferably 1.0 times or more the peak intensity of the (100)plane. In this case, separation performance of the ERI membrane 45 canbe further improved.

The peak intensity refers to a value obtained by subtracting abackground value from a measured value. An X-ray diffraction pattern canbe obtained by irradiation of CuKα-rays to the membrane surface of theERI membrane 45 using an X-ray diffraction apparatus (manufactured byRigaku Corporation, model Miniflex600). Irradiation conditions are setas follows: the X-ray output is 600 W (tube voltage: 40 kV, tubecurrent: 15 mA), the scan speed is 0.5 degrees/min, the scan step is0.02 degrees, and an Ni foil having a thickness of 0.015 mm is used as aCuKβ-ray filter. A peak of the (002) plane is observed around 2 θ=12degrees, and a peak of the (100) plane is observed around 2 θ=8 degrees.

Method for Manufacturing Membrane Structure 42

1. Production of Porous Support 44

A compact is formed by molding a ceramic material into a desired shapeusing an extrusion molding method, a press molding method, a castmolding method, or the like.

Then, the porous support 44 is formed by firing (900° C. to 1450° C.,for example) the compact. The porous support 44 may have an average poresize of 0.01 μm or more and 5 μm or less.

In production of a porous support 44 that has a multilayer structure, aslurry that contains a ceramic material is applied to a surface of afired compact through filtration or the like, and thereafter the compactis fired.

2. Production of Seed Crystals

A starting material solution is prepared by dissolving and dispersingT-atom sources, such as a silicon source, an aluminum source, and aphosphorus source, and a structure-directing agent (SDA) in pure water.It is preferable that at least two of Si, Al, and P are contained as Tatoms, and it is more preferable that at least Al, P, and 0 arecontained, in terms of improving the crystallinity of ERI crystals.Colloidal silica, fumed silica, tetraethoxysilane, sodium silicate, orthe like can be used as a silicon source, for example. Aluminumisopropoxide, aluminum hydroxide, sodium aluminate, alumina sol, or thelike can be used as an aluminum source, for example. Phosphoric acid,sodium dihydrogen phosphate, ammonium dihydrogen phosphate, or the likecan be used as a phosphorus source, for example.N,N,N′,N′-tetramethyldiaminohexane, cyclohexylamine, or the like can beused as a structure-directing agent, for example.

Next, the starting material solution is introduced into a pressurevessel, and hydrothermal synthesis (150° C. to 200° C., 10 to 60 hours)is performed to synthesize ERI crystals.

Then, ERI seed crystals (seed crystals having an ERI structure) areprepared by adjusting the size of the ERI crystals to an extent thatportions of the ERI crystals are locked to openings of pores formed inthe surface of the porous support 44. If an average particle size of thesynthesized ERI crystals is 0.3 times or larger and is smaller than 5times an average pore size of an applied surface of the porous support,these ERI crystals can be directly used as ERI seed crystals (seedcrystals having an ERI structure) as a result of dispersing the ERIcrystals. If the average particle size of the synthesized ERI crystalsis larger than 0.3 times the average pore size of the applied surface ofthe porous support, ERI seed crystals may be produced by introducing theERI crystals into pure water, and deflocculating and crushing the ERIcrystals using a ball mill or the like so that the average particle sizethereof falls within the above-described range. In crushing, the size ofERI seed crystals can be adjusted by changing the crushing time. Theshape of the ERI seed crystals is not particularly limited, and may be ahexagonal column-like shape, a hexagonal plate-like shape, a cylindricalshape, a circular plate-like shape, an irregular shape, or the like, butan isotropic shape is more preferable. The average particle size of theERI seed crystals is preferably 0.3 to 5 times the average pore size ofthe applied surface of the porous support, and is more preferably 0.5 to3 times the average pore size thereof.

3. Formation of ERI Membrane 45

A seed crystal dispersion solution is prepared by dispersing the ERIseed crystals in water, an alcohol such as ethanol or isopropanol, or asolvent obtained by mixing water and an alcohol.

Then, as a result of filtering the seed crystal dispersion solution ontothe surface of the porous support 44, the ERI seed crystals are attachedto the surface of the porous support 44. At this time, the ERI seedcrystals are locked to openings of pores formed in the surface of theporous support 44, and the ERI seed crystals are arranged on the surfaceof the porous support 44.

Then, a starting material solution is prepared by dissolving anddispersing T-atom sources, such as a silicon source, an aluminum source,and a phosphorus source, and a structure-directing agent (SDA) in purewater.

Then, the porous support 44 to which the ERI seed crystals are attachedis immersed in the starting material solution, and hydrothermalsynthesis (150° C. to 190° C., 10 to 60 hours) is performed. At thistime, the ERI seed crystals arranged on the surface of the poroussupport 44 undergo crystal growth such that a-planes of the ERIstructure are adjacent to each other, and therefore joining betweenstanding ERI crystals 46 is facilitated and the ERI membrane 45 isformed as shown in FIG. 3.

Specifically, if the molar ratio H₂O/T atom (H₂O/T atom ratio) is 30 orhigher and the molar ratio N atom in the SDA/T atom (N atom in the SDA/Tatom ratio) is 0.7 to 1.5, it is possible to cause crystal growth of theERI seed crystals such that a-planes of the ERI structure are joined toeach other. If the molar ratio H₂O/T atom is lower than 30, ERI crystalsare generated in the starting material solution during synthesis of themembrane, and the seed crystals on the porous support may be unlikely togrow and it may be difficult to form the membrane. Furthermore, ERIcrystals generated in the starting material solution attach to thesurface of the porous support to which the seed crystals are applied,and the ERI crystals are not oriented to the c-plane, and thereforeseparation performance may be degraded. The H₂O/T atom ratio ispreferably 60 or higher, and more preferably 120 or higher. If the molarratio N atom/T atom in the SDA is higher than 1.5, ERI crystals aregenerated in the starting material solution during synthesis of themembrane, and it may be difficult to form the membrane. If the molarratio N atom in the SDA/T atom is lower than 0.7, ERI crystals are notoriented to the c-plane, and therefore separation performance may bedegraded. The N atom in the SDA/T atom ratio is preferably 0.9 to 1.1.

Dehydration Method

A dehydration method according to the present invention is a method forselectively separating water from the mixture 11 containing water bymaking a pressure difference between opposite surfaces of the ERImembrane 45.

Specifically, the mixture 11 is supplied to the space 4S on the supplyside of the ERI membrane 45 so that the mixture 11 comes into contactwith one surface of the ERI membrane 45, and thereafter the pressure inthe space 4T on the permeation side of the ERI membrane 45 is reduced,whereby water is selectively caused to permeate through the ERI membrane45 and is separated.

In the dehydration method according to the present invention, the ERImembrane 45 that has high durability against water is used as theseparation membrane, and therefore dehydration performance can bemaintained over a long period of time.

Note that, if the mixture 11 is supplied in the form of a liquid, apervaporation method can be used, and if the mixture 11 is supplied inthe form of gas or supercritical gas, a vapor permeation method can beused.

If the pervaporation method is used, the pressure in the space 4S on thesupply side of the ERI membrane 45 is not particularly limited, but ispreferably atmospheric pressure. The pressure in the space 4T on thepermeation side of the ERI membrane 45 is not particularly limited, butis preferably 8×10⁴ Pa or less, more preferably 1×10⁻² to 5×10⁴ Pa, andparticularly preferably 1×10⁻⁴ to 2×10⁴ Pa. The temperature of themixture 11 is not particularly limited, but is preferably 50° C. to 160°C., and more preferably 60° C. to 150° C. Thus, water can be separatedfrom the mixture 11 at a low temperature, and therefore separation canbe performed without using much energy. If the temperature of themixture 11 is higher than 160° C., the energy cost may increase, and ifthe temperature is lower than 50° C., the separation speed may decrease.

If the vapor permeation method is used, the pressure in the space 4S onthe supply side of the ERI membrane 45 is not particularly limited, butis preferably 1×10⁵ to 2.5×10⁷ Pa, and a higher pressure is morepreferable from the standpoint of the separation speed. If the pressuredifference between the supply side space 4S and the permeation sidespace 4T is 2.5×10⁷ Pa or more, the ERI membrane 45 may be damaged orgas-tightness may be degraded. The pressure in the space 4T on thepermeation side of the ERI membrane 45 is only required to be lower thanthe pressure in the supply side space 4S, but is preferably 8 x 10 ⁴ Paor less, more preferably 1×10⁻² to 5×10⁴ Pa, and particularly preferably1×10⁻⁴ to 2×10⁴ Pa. The temperature of the mixture 11 is notparticularly limited, but is preferably 50° C. or higher, morepreferably 100° C. to 400° C., and particularly preferably 100° C. to200° C., in terms of energy cost. If the temperature of the mixture 11is lower than 50° C., the separation speed may decrease. If thetemperature of the mixture 11 is higher than 400° C., the membrane maybe degraded.

The water permeation flux of the ERI membrane 45 at 50° C. is preferably1 kg/(m²·h) or more, more preferably 1.5 kg/(m²·h) or more, andparticularly preferably 2 kg/(m²·h) or more, in terms of improvingdehydration performance.

The water permeation flux can be determined by supplying pure waterheated to 50° C. to the space 4S on the supply side of the ERI membrane45, reducing the pressure in the space 4T on the permeation side of theERI membrane 45 to 50 Torr, and collecting water vapor permeated throughthe ERI membrane 45.

Water selectivity of the ERI membrane 45 as determined using an aqueousethanol solution at 50° C. is preferably 10 or higher, more preferably20 or higher, and particularly preferably 50 or higher, in terms ofimproving water permeation selectivity in a dehydration operation. Waterselectivity can be determined as a ratio (water concentration inpermeating substance/ethanol concentration in permeating substance), inwhich “water concentration in permeating substance” is the concentration(mass %) of water and “ethanol concentration in permeating substance” isthe concentration (mass %) of ethanol in a liquid that is obtained bysupplying a 50 mass % aqueous ethanol solution heated to 50° C. to thespace 4S on the supply side of the ERI membrane 45, reducing thepressure in the space 4T on the permeation side of the ERI membrane 45to 50 Torr, and collecting the vapor permeated through the ERI membrane45.

Other Embodiments

In the above-described embodiment, the structure of the separationvessel 40 is described with reference to FIGS. 1 and 2, but theseparation vessel is not limited to this structure. The separationvessel 40 is only required to have a structure that includes the housingportion 41 and the membrane structure 42 (the ERI membrane 45 and theporous support 44), and is configured to be capable of carrying out theabove-described dehydration method.

In the above-described embodiment, the dehydration apparatus 100includes, as one example of a “pressure changing apparatus”, thepressure reducing apparatus 60 that reduces the pressure in thepermeation side space 4T, but the dehydration apparatus 100 may includea pressure increasing apparatus that increases the pressure in thesupply side space 4S, instead of or in addition to the pressure reducingapparatus 60.

EXAMPLES

Examples of the present invention will be described below. However, thepresent invention is not limited to the examples described below.

Example 1

1. Production of Porous Support

A monolith-shaped compact having a plurality of through holes was formedwith use of a green body containing an alumina raw material and anextrusion molding method, and then was fired.

Then, a porous layer including alumina as a main component was formed onsurfaces of through holes of the fired compact, and the resultingcompact was fired again to form a porous support. A surface of a portionof the porous support on which a membrane was to be formed had anaverage pore size of 65 to 110 nm.

2. Production of Seed Crystals

A starting material solution having a composition of 1 Al₂O₃:1.3 P₂O₅:1.4 SDA: 130 H₂O was prepared by dissolving, in pure water, aluminumisopropoxide, 85% phosphoric acid, andN,N,N′,N′-tetramethyldiaminohexane (TMHD), which is astructure-directing agent (SDA).

Then, the starting material solution was introduced into a pressurevessel, and hydrothermal synthesis (195° C., 30 hours) was performed.

Then, crystals obtained through hydrothermal synthesis were collectedand sufficiently washed with pure water, and then were completely driedat 65° C.

Then, a crystal phase was checked through X-ray diffraction measurement,and it was confirmed that the obtained crystals were ERI crystals.

Then, ERI seed crystals were produced by introducing the synthesized ERIcrystals into pure water in an amount of 10 to 20 mass % and crushingthe ERI crystals using a ball mill for 7 days. External forms of the ERIseed crystals were checked using a SEM (scanning electron microscope),and it was found that the obtained ERI seed crystals had irregularforms, the particle size of the seed crystals was 0.01 to 0.3 μm, andthe average particle size of the seed crystals was about 0.2 μm.

3. Formation of ERI Membrane

A seed crystal dispersion solution was prepared by dispersing the ERIseed crystals in ethanol.

Then, as a result of filtering the seed crystal dispersion solutionthrough cells of the porous support, the ERI seed crystals were attachedto inner surfaces of the cells of the porous support.

Then, a starting material solution having a composition of 1 Al₂O₃:2.1P₂O₅:2.8 SDA:1340 H₂O was prepared by dissolving, in pure water,aluminum isopropoxide, 85% phosphoric acid, and TMHD, which is astructure-directing agent (SDA). In the starting material solution ofExample 1, the molar ratio H₂O/T atom (H₂O/T atom ratio) was 220, andthe molar ratio N atom in the SDA/T atom (N atom in the SDA/T atomratio) was 0.9.

Then, the porous support to which the ERI seed crystals were attachedwas immersed in the starting material solution, and hydrothermalsynthesis (160° C., 30 hours) was performed to synthesize an ERImembrane.

Then, the synthesized ERI membrane was sufficiently washed with purewater, and was completely dried at 90° C.

Then, SDA was burned off by heating the ERI membrane at 450° C. for 50hours so that pores extended through the ERI membrane.

4. Separation Durability Test

A separation test was performed using the ERI membrane obtained asdescribed above and a separation test apparatus shown in FIG. 1.

First, a 50 mass % aqueous ethanol solution heated to 50° C. wascirculated using a circulation pump to supply the aqueous ethanolsolution to a supply side space of a separation vessel.

Then, the pressure on the porous support side of the ERI membrane(permeation side space) was reduced to 50 Torr using a vacuum pump,while being controlled by a pressure controller, and vapor permeatedthrough the ERI membrane was collected in a liquid nitrogen trap. Theethanol concentration in a liquid that was collected in the liquidnitrogen trap was taken as the “ethanol concentration before hot watertreatment”.

Then, the ERI membrane was removed from the separation test apparatus,was immersed in pure water in a pressure vessel, and hot water treatment(130° C., 50 hours) was performed.

Then, the ERI membrane was sufficiently washed with pure water, and wascompletely dried at 200° C.

The above-described separation test was performed again using the ERImembrane subjected to the hot water treatment. The ethanol concentrationin the liquid that was collected in the liquid nitrogen trap was takenas the “ethanol concentration after hot water treatment”.

As a result, it was found that the “ethanol concentration after hotwater treatment” was not higher than 1.5 times the “ethanolconcentration before hot water treatment”.

Thus, it was found that the obtained ERI membrane had high stabilityagainst hot water and its dehydration performance was unlikely todegrade.

5. Microstructure Evaluation

In an X-ray diffraction pattern obtained by irradiation of X-rays to themembrane surface of the ERI membrane, the peak intensity of a (002)plane (c-plane) was 0.90 times the peak intensity of a (100) plane(a-plane). The outer surface of the ERI membrane and its cross sectionobtained by cutting the ERI membrane in its thickness direction wereobserved using a SEM, and it was confirmed that hexagonal column-shapedERI crystals were oriented to the c-plane (see FIGS. 3 and 4).

Example 2

1. Production of Porous Support

A porous support was produced in the same process as that of Example 1.

2. Production of Seed Crystals

ERI seed crystals were produced in the same process as that of Example1.

3. Formation of ERI Membrane

An ERI membrane was synthesized in the same process as that of Example1, except that the composition of the starting material solution waschanged to 1 Al₂O₃:2.0 P₂O₅:3.0 SDA:210 H₂O, and hydrothermal synthesiswas performed at 170° C. for 50 hours. In the starting material solutionof

Example 2, the H₂O/T atom ratio was 35, and the N atom in the SDA/T atomratio was 1.0.

Then, the synthesized ERI membrane was sufficiently washed with purewater, and was completely dried at 90° C.

Then, SDA was burned off by heating the ERI membrane at 450° C. for 50hours so that pores extended through the ERI membrane.

4. Separation Durability Test

A separation test was performed in the same manner as that of Example 1,using the ERI membrane obtained as described above. The ethanolconcentration in the liquid that was collected in the liquid nitrogentrap was taken as the “ethanol concentration before hot watertreatment”.

Hot water treatment was performed in the same manner as that of Example1, and the separation test was performed again. The ethanolconcentration in the liquid that was collected in the liquid nitrogentrap was taken as the “ethanol concentration after hot water treatment”.

As a result, it was found that the “ethanol concentration after hotwater treatment” was not higher than 1.5 times the “ethanolconcentration before hot water treatment”.

Thus, it was found that the obtained ERI membrane had high stabilityagainst hot water and its dehydration performance was unlikely todegrade.

5. Microstructure Evaluation

In an X-ray diffraction pattern obtained by irradiation of X-rays to themembrane surface of the ERI membrane, the peak intensity of a (002)plane was 0.51 times the peak intensity of a (100) plane. The outersurface of the ERI membrane and its cross section obtained by cuttingthe ERI membrane in its thickness direction were observed using a SEM,and it was confirmed that hexagonal column-shaped ERI crystals wereoriented to the c-plane.

Comparative Example 1

1. Production of Porous Support

A porous support was produced in the same process as that of Example 1.

2. Production of Seed Crystals

LTA seed crystals were produced by introducing crystals of acommercially available NaA type zeolite (zeolite having an LTAstructure) into pure water in an amount of 10 to 20 mass %, and crushingthe crystals using a ball mill for 6 hours. External forms of the LTAseed crystals were checked using a SEM (scanning electron microscope),and it was found that the obtained LTA seed crystals had irregularforms, the particle size of the seed crystals was 0.01 to 0.3 μm, andthe average particle size of the seed crystals was about 0.2 μm.

3. Formation of LTA Membrane

A seed crystal dispersion solution was prepared by dispersing the LTAseed crystals in pure water.

Then, as a result of filtering the seed crystal dispersion solutionthrough cells of the porous support, the LTA seed crystals were attachedto inner surfaces of the cells of the porous support.

Then, a starting material solution having a composition of 1 Al₂O₃:4SiO₂:40 Na₂O:1600 H₂O was prepared by dissolving, in pure water,aluminum sulfate, silica sol, and sodium hydroxide.

Then, the porous support to which the LTA seed crystals were attachedwas immersed in the starting material solution, and hydrothermalsynthesis (100° C., 10 hours) was performed to synthesize an LTAmembrane.

Then, the synthesized LTA membrane was sufficiently washed with purewater, and was completely dried at 90° C.

4. Separation Durability Test

A separation test was performed in the same manner as that of Example 1,using the LTA membrane obtained as described above. The ethanolconcentration in the liquid that was collected in the liquid nitrogentrap was taken as the “ethanol concentration before hot watertreatment”.

Hot water treatment was performed in the same manner as that of Example1, and the separation test was performed again. The ethanolconcentration in the liquid that was collected in the liquid nitrogentrap was taken as the “ethanol concentration after hot water treatment”.

As a result, it was found that the “ethanol concentration after hotwater treatment” was approximately the same as that in the suppliedaqueous ethanol solution, and was higher than 1.5 times the “ethanolconcentration before hot water treatment”.

5. Microstructure Evaluation

The outer surface of the LTA membrane was observed using a SEM, and itwas confirmed that LTA dissolved and was lost through the hot watertreatment.

Thus, it was found that the obtained LTA membrane had low stabilityagainst hot water and its dehydration performance was likely to degrade.

1. A dehydration method for selectively separating water from a mixturethat contains water, using a zeolite membrane having an ERI structure,the method comprising: a step of supplying the mixture to a supply sidespace of the zeolite membrane having an ERI structure; and a step ofmaking a pressure difference between the supply side space and apermeation side space of the zeolite membrane having an ERI structure.2. The dehydration method according to claim 1, wherein the zeolitemembrane having an ERI structure contains at least two elements out ofSi, Al, and P.
 3. The dehydration method according to claim 2, whereinthe zeolite membrane having an ERI structure contains at least Al, P,and O.
 4. The dehydration method according to claim 1, wherein thezeolite membrane is formed on a porous support.
 5. A dehydrationapparatus comprising: a separation vessel configured to include a poroussupport, a zeolite membrane having an ERI structure that is formed onthe porous support, and a housing portion that is sectioned into asupply side space of the zeolite membrane having an ERI structure and apermeation side space of the zeolite membrane having an ERI structure;and a pressure changing apparatus configured to increase a pressure inthe supply side space and/or reduces a pressure in the permeation sidespace.
 6. The dehydration apparatus according to claim 5, wherein thezeolite membrane having an ERI structure contains at least two elementsout of Si, Al, and P.
 7. The dehydration apparatus according to claim 6,wherein the zeolite membrane having an ERI structure contains at leastAl, P, and O.